Device for punctual measurement of a radiofrequency magnetic field with constant amplitude and frequency

ABSTRACT

The invention concerns a device for measuring a radiofrequency magnetic field, characterised in that it comprises on a common support: a coil forming a primary reception antenna; a voltage-controlled oscillator, monitored so that its output frequency depends on the amplitude of the magnetic field received on the coil forming primary antenna; an absorption modulation load connected to the oscillator output; a secondary antenna sensitive to the magnetic field influenced by the absorption modulation resulting from the power supply load by the voltage-controlled oscillator, and means for operating the available signal on the secondary antenna.

FIG. 1 illustrates one embodiment of a system including: an induction coil 1 of a contact-free card reader; and a card 10 forming a support, carrying the components used in a measurement device.

FIG. 2 illustrates an example embodiment of a chip that includes a primary antenna AP, a voltage controlled oscillator VCO and a modulation load MA.

This invention relates to the field of measuring a magnetic field.

More precisely, this invention relates to a device for punctual measurement of a radiofrequency magnetic field with constant amplitude and frequency.

This invention is particularly but not exclusively applicable to contact-free card readers and particularly to readers for components with integrated coils.

The purpose of this invention is to propose a device capable of measuring the magnetic field generated by devices such as contact-free card readers, and particularly for systems with fields of about 10⁻³ Tesla.

To the best knowledge of the inventor, no device planned at the moment is fully satisfactory for this type of application.

The above-mentioned purpose is achieved within the scope of this invention by means of a device for measuring the radiofrequency magnetic field characterized by the fact that it comprises the following, on a common support:

-   -   a coil forming a primary reception antenna,     -   a voltage controlled oscillator, monitored so that its output         frequency depends on the amplitude of the magnetic field         received on the coil forming the primary antenna;     -   an absorption modulation load connected to the oscillator         output;     -   a secondary antenna sensitive to the magnetic field influenced         by the absorption modulation resulting from the power supply to         the load by the voltage controlled oscillator, and     -   means for using the available signal on the secondary antenna.

According to another advantageous characteristic of the invention, the support consists of a card made of non-magnetic material.

According to another advantageous characteristic of this invention, the device comprises a chip integrated on the support that includes the primary antenna, the voltage controlled oscillator and the absorption modulation load.

Other characteristics, purposes and advantages will become clear after reading the following detailed description with reference to the attached drawings given as a non-limitative example and on which FIG. 1 diagrammatically shows a device according to this invention, and FIG. 2 shows a detailed view of the assembly formed by the primary antenna, the voltage controlled oscillator and the modulation load.

The attached FIG. 1 diagrammatically shows the induction coil 1 of a contact-free card reader and a card 10 forming a support, made of a non-magnetic material carrying all components used in the measurement device according to this invention.

Typically, but not limitatively, the induction coil 1 is adapted to emit a radiofrequency magnetic field with an amplitude of about 10⁻³ Tesla at a frequency of 13.56 MHz.

Typically, but not limitatively, the dimensions of the support forming the card 10 are 55×85 mm.

The device according to this invention, for which the structure will be described in more detail later, consists firstly of a small sensor 20 inserted in the magnetic field of the induction coil 1 and that is capable of transmitting information about the captured energy to the exterior, and secondly means 30 for processing this information, to amplify, calculate the value of the magnetic field at the precise location at which the said sensor 20 is located, and display it as a direct readout on a display unit 36.

The sensor 20 is composed of:

-   -   firstly, a small electronic chip 21 (for example 1.4 mm side         dimension) comprising an integrated coil forming a primary         reception antenna AP, a VCO or voltage controlled oscillator,         the frequency of which depends on the magnetic field in which         the chip 21 is located, and an absorption modulation device MA         to transmit frequency information from the said VCO to the         exterior;     -   secondly, an antenna 22 composed of a few turns, for example 2         turns, and an electrical conductor wound tightly around the         electronic chip 21 forming a secondary antenna.

Preferably, the absorption modulation device MA is formed from a resistance Rmod connected to the output from the voltage controlled oscillator. Furthermore, this voltage-controlled oscillator is preferably adapted to emit a signal with a frequency of between about 500 and 15000 Hz.

The magnetic field/frequency transfer function of the voltage-controlled oscillator is preferably calibrated in the laboratory.

The usage means 30 is connected to the output from the secondary antenna 22. Thus, the operating means 30 receive a signal for which the basic frequency corresponds to the induction coil emission frequency and for which the amplitude is modulated at the same rate as the frequency generated by the voltage controlled oscillator, this frequency being representative of the amplitude of the magnetic field received on the chip 21.

There are several possible variants of the means for using the signal available on the secondary antenna 22.

According to the particular but non-limitative embodiment illustrated in FIG. 1 attached, these operating means comprise:

-   -   an amplitude demodulator 31 connected to the output from sensor         20, more precisely to the output from the secondary coil 22, the         said demodulator 31 outputs frequency information at a low level         of the order of a few millivolts,     -   an RC 32 filter connected to the output from the demodulator 31;     -   an amplifier assembly 33 connected to the output from filter 32,         with a sufficient gain to trigger     -   a comparator system 34 providing a logical level frequency         output signal compatible with     -   processing means 35 preferably using a standard microcontroller,         for example such as Motorola's MC68HC11 microcontroller.

This microcontroller 35 is programmed such that it calculates the value of the quasi punctual magnetic field available on chip 21, starting from the frequency information and using the transfer function mentioned above.

The microcontroller program 35 also manages the display of the value of the magnetic field, preferably calculated in gauss, in real time on a display 36. This display is preferably a liquid crystal display and is integrated on the card support 10. However, if required, the display may be placed elsewhere on a module external to the card support 10.

This invention may be used for applications in the field of measuring radiofrequency magnetic fields with constant amplitude and frequency, and particularly the adjustment of induction coils 1 used on very close contact-free card readers.

FIG. 2 attached illustrates an example embodiment of the chip 21 that includes the primary antenna AP, the voltage controlled oscillator VCO and the modulation load MA.

In this example, the alternating voltage available at the output from the primary antenna AP is rectified by a rectifier R and filtered by a capacitor C to output a DC voltage VDD. This voltage is input to the voltage-controlled oscillator VCO and controls the VCO through a resistive divider bridge R1/R2 powered by VDD. The modulation resistance Rmod is placed in series with the main conduction path of a switching transistor T at the terminals of the power supply VDD. The transistor T is controlled through the output from the VCO.

The DC current that passes through the modulation resistance Rmod is chopped at the same frequency as the VCO, which depends on the amplitude of the field received on the primary antenna AP. The alternating current at 13.56 MHz that passes through the primary antenna AP is modulated accordingly.

The primary antenna AP and secondary antenna 22 are closely coupled, this current induces an electromotive force (EMF) in the secondary antenna 22. This EMF is additional to the EMF induced by the induction coil 1 of the reader.

Therefore, the resulting EMF is an alternating voltage at 13.56 MHz slightly modulated in amplitude at the frequency of the VCO.

The modulation resistance Rmod must be sufficiently low to generate a current providing a detectable EMF, but it must not be too low to avoid degrading the retrieved DC voltage supplying power to the VCO. For example it can be 20 KΩ.

Obviously, this invention is not limited to the particular embodiment that has just been described, but includes any variant conform with its spirit.

Any device capable of consuming electrical power can be used for modulation. For example, the resistance could be replaced by a current generator. 

1. Device for measuring a radiofrequency magnetic field, characterised by the fact that it comprises the following, on a common support (10): a coil forming a primary reception antenna, a voltage controlled oscillator, monitored so that its output frequency depends on the amplitude of the magnetic field received on the coil forming the primary antenna; an absorption modulation load connected to the oscillator output; a secondary antenna (22) sensitive to the magnetic field influenced by the absorption modulation resulting from the power supply to the load by the voltage controlled oscillator, and means (30) for using the available signal on the secondary antenna (22).
 2. Device according to claim 1 characterised by the fact that the support (10) consists of a card made of non-magnetic material.
 3. Device according to either of claims 1 or 2, characterised by the fact that it comprises a chip integrated on the support, including the primary antenna, the voltage controlled oscillator and the absorption modulation load.
 4. Device according to one of claims 1 to 3, characterised by the fact that it also comprises a display unit (36).
 5. Device according to one of claims 1 to 4, characterised by the fact that the absorption modulation load is formed from a resistance.
 6. Device according to one of claims 1 to 4, characterised by the fact that the absorption modulation load comprises a current generator.
 7. Device according to one of claims 1 to 6, characterised by the fact that the secondary antenna is composed of a few turns of an electrical conductor wound tightly around the assembly formed by the primary coil, the voltage controlled oscillator and the absorption modulation load.
 8. Device according to one of claims 1 to 7, characterised by the fact that the operating means comprise a demodulator (31) connected to the output from the secondary antenna (22).
 9. Device according to claim 8, characterised by the fact that it comprises a filter (32) at the output from the demodulator.
 10. Device according to one of claims 1 to 9, characterised by the fact that the operating means comprise a high gain amplifier (33) to trigger a comparator system (34) outputting a logical level frequency output signal.
 11. Device according to one of claims 1 to 10, characterised by the fact that the operating means comprise a microcontroller programmed to calculate information representative of the value of the magnetic field starting from the available information at the output from the secondary antenna (22).
 12. Device according to one of claims 1 to 11, characterised by the fact that the coil (AP) forming primary antenna is input to a rectifier (R) that supplies a DC voltage (VDD) that supplies power to the voltage controlled oscillator (VCO), controls it and supplies the modulation load (MA) connected in series with a switching transistor (T) controlled through the output from the voltage controlled oscillator (VCO). 